The invention described herein relates to adjusting light beams in optical switches. In particular, methods and apparatus for achieving desired reflector positioning in optical switches through the indirect measurement of light beams.
As is well known, fiber optic technology is a rapidly growing field with vastly expanding commercial applicability. As with all technologies, fiber optic technology is faced with certain practical difficulties. In particular, implementation of efficient coupling between an input optical elements and output optical elements in the optical switching elements of an optical network is a significant consideration of designers, manufacturers, and users of optical systems. Optical systems use light beams, usually laser-generated, to carry various types of information. Commonly, these light beams travel through optical fibers or through other optical elements such as optical switches. In optical systems, light beams are directed through complex optical paths with the assistance of optical switching elements. As it happens, losses of optical power in switching elements are a significant concern.
Fiber-to-fiber coupling in an optical switch should be efficient to avoid unnecessary losses in optical power. Coupling efficiency is especially important in optical systems where light beams are subject to reflection as part of the optical switching process. If too much light is lost due to alignment and reflection errors in the switch, the light output from the switch might be insufficient for its intended purpose.
When efficiently coupling, a light beam travelling through an optical switch enters an output optical fiber so that the amount of light transmitted through the fiber is maximized. The most efficient coupling between an optical beam and a fiber occurs when the light beam is centrally positioned on the core of the fiber (on the fiber center) and when the beam enters the fiber at an acceptable angle of entry. Such an acceptable angle of entry is dependent on fiber characteristics, such as, fiber type, size, and cladding. When the light beam enters an output fiber at an acceptable angle of entry and at a central position, an optimal amount of light is transmitted through the output fiber.
However, once positioned on the fiber center at an acceptable angle, the light beam does not always remain in place. Operating conditions which may cause the system to suffer a shock or vibration, for example, can cause the physical components of the optical system to shift, causing the light beam to be offset from the fiber center. Other factors may also cause the light beam position and angle to shift. Changing environmental conditions may result in beam variance from the original position. For example, thermal expansion of a fiber may result in a shift in beam position. Thermal effects may also cause subtle distortion of switch components (like reflector surfaces) resulting in changes in beam angle and position. These and other effects can result in reduced coupling efficiency between the light beam and output fiber. A system and method for efficient coupling must be able to correct offsets due to vibration, thermal expansion, or other causes. Moreover, there can be occasions when it is desirable to intentionally reduce the coupling efficiency between the light beam and output fiber. For example, coupling efficiency can be reduced in order to attenuate optical power in a light beam. The system and method for efficient coupling must be able to accommodate these needs as well.
Previous attempts to solve the foregoing problems have met with mixed success. FIG. 1 shows the inner workings of a conventional optical switching array 1. Briefly, the components include an array of input optical fibers 10, which are positioned and aligned using an input block 11. The light beams exiting the optical fibers 10 are directed through an input lens array 12. The lens array 12, collimates and focuses the optical beams (shown here by a single example optical beam B) such that they are directed onto a first movable reflector array 13, which directs the beams onto a second movable mirror array 14 such that the beams are directed through an output lens array 15 into output channels which correspond to optical fibers 20, which are aligned and positioned in an output fiber block 16. The reflectors of the movable reflector arrays 13, 14 are oriented to direct the optical beams from selected input fibers into selected output fibers. By correctly orienting the mirrors, beams are switched from input fiber to output fiber in order to accomplish the switching function of the switch. The orientation of the reflectors of the movable mirror arrays 13, 14 is controlled by control circuitry (not shown), which moves the individual reflectors of the reflector arrays 13, 14 to accomplish the switching function of the optical switches discussed herein.
A typical example of a movable reflector array 13, 14 is a Micro Electro-Mechanical System (MEMS) reflector array constructed of a plurality of micro-scale movable reflectors formed on a monolithic silicon substrate. Such devices are manufactured by, for example, Analog Devices of Cambridge, Mass., or MCNC of Research Triangle Park, North Carolina.
FIG. 2 is a block diagram illustrating one implementation used to optimize mirror orientation in an optical switch to obtain maximum beam power in an output light beam. The collimator and reflector elements depicted in FIG. 1 are schematically depicted as the switch 17. Optical beams are input into the switch 17 through the input fibers 10. The output optical beams are received by the output fibers 20. Each input fiber 10 is equipped with a detector element 21 that monitors optical power. Similarly, each output fiber 20 includes a similar detector element 22. The outputs from the input detectors 21 and output detectors 22 directly measure optical power in the light beams to position the switch reflectors in order to optimize power. The light detectors 21, 22 directly measure input power and output power and uses this information to adjust the reflectors of the switch path in accordance with power optimization algorithms to maximize the fiber coupled output power. Examples of such power optimization techniques using directly measured light beams is described in detail in the U.S. Patent Application entitled: xe2x80x9cFeedback Stabilization of a Loss Optimized Switchxe2x80x9d, filed on Apr. 30, 2000, Ser. No. 09/548,587, which is hereby incorporated by reference. Although such systems are satisfactory for their intended purpose, improvements can be made.
A disadvantage of such conventional direct measurement devices is that each fiber 10, 20 requires a detector element (e.g., 21, 22) so that input power can be directly compared to output power. Consequently, in a switch having, for example, 256 input and output fibers, 512 such detectors are required (one for each input fiber and each output fiber). Still other approaches use pairs of quadrature detectors for each fiber. Because each quadrature detector comprises four photodetectors, these solutions require eight photodetectors and their supporting circuitry (including amplifiers) per light beam. In addition to the large number of detectors needed by such implementations, the detectors themselves can be quite large, thereby substantially increasing the size of such switches. Also, each splitter/tap is an expensive component requiring individual alignment during manufacture. These factors can significantly increase the cost of such switches. Also, existing switches use a test light beam which is propagated in the direction opposite that of an output beam. This counter propagating light beam is used to align and adjust the beams of the switch and also prevent xe2x80x9cfalse positivexe2x80x9d readings generated by stray light in the switch. The need for a test beam increases cost and complicates the system. As a result, it is desirable to develop methods and apparatus for optimizing light beam power in optical switches which do not rely on such direct measurements of optical power and do not require a counter propagating light beam to align and adjust the beams of a switch and does not generate false positive readings.
Therefore, an improved system and method for adjusting light beams in an optical switch using indirect measurement of light beams is needed.
In accordance with the principles of the present invention, an apparatus and method for indirectly monitoring and adjusting optical beams by the detection and indirect measurement of at least one monitoring beam and using such measurements to adjust the reflectors of an optical switch in order to obtain output beams having the desired optical characteristics is disclosed.
An embodiment of the present invention splits a light beam into a working beam and at least one monitoring beam. The at least one monitoring beam is measured and used to adjust reflectors such that the working beam is adjusted and maintained having the desired optical properties without the need to interfere with the working beam signal.
A method embodiment for indirectly measuring and adjusting light beams output from an optical switch comprises receiving, by the optical switch, an input light beam, switching of the input light beam, such that the input light beam is optically coupled to any of a plurality of selected output channels, and, after switching, splitting the light beam into at least one monitor beam and a working beam, detecting the optical power of at least one monitor beam, and based on the detected optical power of the at least one monitor beam, adjusting the working beam until it exhibits the desired optical characteristics, and outputting the working beam from the switch. Further method embodiments include adjusting the working beam until it exhibits optimal power. Still other method embodiments direct the at least one monitor beam to a position remote from the working beam where it is detected. Still other method embodiments split the input beam such that the at least one monitor beam and the working beam are substantially parallel to each other.
An optical switch embodiment comprises an array of optical input channels each capable of carrying an associated input light beam, an array of output channels, an array of beam monitoring elements, a switching array for coupling selected input channels to selected output channels enabling the switching of each input light beam to any one of a plurality of output channels, and a beam splitter optically interposed between the switching array and the array of output channels, and positioned to receive light beams from the switching array such that a portion of the light from the light beams is directed as monitor beams onto the array of beam monitoring elements, and such that another portion of the light from the light beams is directed to the array of output channels as working beams. Beam monitoring elements measure the monitor beams to provide optical information used for adjusting the switching array such that the working beams enter the output channels having desired optical characteristics.
Yet another embodiment comprises an array of input channels each capable of carrying an associated light beam, an array of output channels, an array of beam monitoring elements, a switching array including reflector arrays for switching selected light beams received from the input channels into any of a selected plurality of output channels as output beams, rhomboid prism assemblies positioned to receive the output beams from the switching array such that a portion of the light from the plurality of output beams is reflected in the form of monitor beams, and such that another portion of the light from the plurality of output beams passes through the rhomboid prism assemblies as working beams, wherein the monitor beams are reflected such that the monitor beams and the working beams emerge from the rhomboid prism assemblies as substantially parallel beams, wherein the working beams are directed into the plurality of output channels and wherein the monitor beams are directed into the beam monitoring elements wherein they are measured and used to provide information for adjusting the working beam.
Another embodiment comprises a fiber array having a plurality of input fibers and a plurality of output fibers. The input fibers and output fibers are capable of carrying an associated light beam. The embodiment further comprises an array of beam monitoring elements located physically apart from the fiber array, a switching array comprising, in combination, a plurality of first movable reflectors, a plurality of second movable reflectors, a stationary reflector; and control circuitry operating such that a light beam input into the switching array from an input fiber can be switched to a selected output fiber. The embodiment also includes a beam splitter interposed between the switching array and the fiber array such that a portion of the light from the light beams exiting the switching array is reflected by the beam splitter as monitor beams onto the physically separate beam monitoring elements and such that another portion of the light from the light beams exiting the switching array passes through the beam splitter as working beams which enter the output fibers; and wherein the beam monitoring elements measure the monitor beams and provide optical information to the control circuitry for adjusting the switching array such that the working beams enter the output fibers having the desired optical characteristics.
These and other aspects and advantages of the invention will become apparent from the following detailed description and accompanying drawings which illustrate, by way of example, the principles of the invention.